The present invention relates to a process for recovery of common salt and marine chemicals of high purity in integrated manner, which boosts the viability of such recovery. The process is amenable to a wide range of brine compositions but especially attractive for brine compositions that are low in sulphate content and yield impure salt when the conventional process of solar salt production is followed.
Certain components of brine have industrial uses. Common salt, apart from being an essential dietary component, is a basic raw material for the manufacture of a wide variety of industrial chemicals viz. sodium carbonate (soda ash), sodium hydroxide (caustic soda), and chlorine. Salt is also used in textile, dairy, dyeing, food, fertilizer, paper and pharmaceutical industries. Marine gypsum is used in cement industries and in the preparation of high strength Plaster of Paris. It can also be used as a source of calcium in the preparation of calcium-based siliceous chemicals. Magnesium compounds find applications in agriculture, refractories, pharmaceuticals, rubber, polymer additives and fire retardant. Potash is an essential plant nutrient and chemical grade KCl is used for making other important potash chemicals.
Reference may be made to xe2x80x9cRain Washing of Common Salt Heapsxe2x80x9d by M. P. Bhatt, P. S. Jesulpura and K. Sheshadri, Salt Research and Industry, 10(2), (1974), 13, who have reported that sea salt which is harvested and subjected to rain water washing, has 0.21% w/w Ca, 0.60% w/w sulphate and 0.06% Mg. The salt requires upgradation to reduce the level of calcium and sulphate, especially for use in chloralkali industry.
Reference may be also made to xe2x80x9cFractional Crystallisation of Salts from Sub-soil brinesxe2x80x9d by V. P. Mohandas, S. J. Gohil, and S. D. Gomkale, International Journal of Salt Lake Research 6, (1998), 331, who have reported that sub-soil brines of Gujarat, India, typically yield salt contaminated with 0.30-0.40% w/w Ca, 0.80-1.00% w/w sulfate and 0.20-0.30% Mg after harvesting and washing of heaps with a minimum quantity of water. This makes the salt unacceptable for industrial application.
The authors have attributed the higher Ca impurity in salt produced from sub-soil brine to the inherent composition of the brine.
In the article xe2x80x9cWashing of Strip Mined Rock and Solar Salt at Leslie Salt Corporation, U.S.A. (Symposium on Salt-1, Vol. 1, The Northern Ohio, Geological Society Incorporation, Cleveland, (1961), 449-464), Woodhill has stated that a washery is useful for reducing calcium, magnesium and sulphate impurities in solar salt. The main disadvantage of the method is that there are 10-15% losses, high capital investment is involved, and the maximum level of reduction of Ca is 70%.
In the article xe2x80x9cManufacture of Salt by Series Feeding Systemxe2x80x9d by R. B. Bhatt, R. M. Bhatt, U. V. Chitnis, P. S. Jesulpura and K. Sheshadri, Salt Research and Industry, 11, (1979), 9, it has been stated that sea salt can be prepared with lower calcium impurity by adopting series feeding method wherein the brine is subjected to fractional crystallization over narrower density ranges, and the salt is harvested between 27.0-29.5xc2x0 Be. The drawbacks of this process are that the yield of pure salt is reduced as it is harvested over a narrower density range, and the salt is more contaminated with magnesium sulphate impurity which can only be satisfactorily removed with the help of a washery. Moreover, as found by the present inventors, series feeding does not yield improved quality salt when sub-soil brine is used.
In their patent application (Indian Patent Application No. 315/DEL/95) entitled xe2x80x9cA Process for the Preparation of Sodium Chloride containing Low Calcium Impurity from Sea Brine in Solar Salt Worksxe2x80x9d, M. H. Vyas, H. N. Shah, J. R. Sanghavi, M. R. Gandhi and R. J. Sanghavi have claimed that calcium can be reduced by up to 70% in the harvested salt through treatment with activated starch solution. The drawbacks of the process are that it is not applicable to subsoil brines, and it is also difficult to implement the process in large-scale commercial production because of the large requirement of starch solution. Another drawback is that magnesium and sulphate impurities are still high.
In their patent application (PCT Application filed, 2001) entitled xe2x80x9cAn improved Process for the Removal of Ca Ions from the brine by Marine Cyanobacteriaxe2x80x9d, S. Mishra, P. K. Ghosh, M. R. Gandhi, A. M. Bhatt and S. A. Chauhan have claimed the production of low Ca salt from sea/sub-soil brine by mopping up Ca in the brine through certain types of marine cyanobacteria. The drawback of the process is that it is not readily amenable to scale up and magnesium and sulphate impurities would continue to pose a problem.
Besides the drawbacks indicated for the above processes, none of them integrate with subsequent marine chemicals recovery and do not in any way improve the composition of bittern and, therefore, the process of such recovery is tedious as described below.
Potassium chloride is produced most commonly from potash deposits (e.g., Strassford deposits of Germany) either by froth flotation technique or by a hot leaching process. Reference may be also made to the process described in World Survey of Potash Resources, The British Sulfur Corporation, London 1985, wherein potash is produced from Dead Sea brine through intermediate formation of carnallite (KCl.MgCl2.6H2O). However, sea water and sub-soil brines such as exist in India yield kainite (KCl.MgSO4.3H2O) double salt instead of carnallite because of the much higher sulphate content of the brine.
Reference may be also made to the paper xe2x80x9cPotassium from Sea Waterxe2x80x94A Daring Venturexe2x80x9d, Chemistry and Industry, Nov. 13, 1971, 1309 by J. Kielland wherein it is stated that Dipycrylamine can be used to precipitate potash directly from sea water. The drawback of the process is the extremely high toxicity of the extracting reagent and the difficulty in recycling the extractant.
Reference may be made to xe2x80x9cManufacture of Potassium chloride and byproducts from Sea Bitternxe2x80x9d by K. Sheshadri et al. published in Salt Research and Industry 7, (April-July 1970), 39-44, wherein bittern is further concentrated in solar pans and, after removing crude salt and Sels"" mixture (mixture of NaCl and MgSO4), mixed salt (NaCl and kainite) is formed in solar pans. Mixed salt is dispersed with high density bittern in proper proportion and heated to a temperature of 110xc2x0 C. when keiserite (MgSO4.H2O) is formed which is separated by filtering the slurry under hot conditions. The filtrate is cooled to ambient temperature, when camallite crystallizes out. Camallite is decomposed with water to get a solid mixture of sodium chloride and potassium chloride while magnesium chloride goes into solution. Solid mixture of potassium chloride and sodium chloride is purified using known techniques to produce pure potassium chloride. The drawbacks of this process are: Mixed Salt (containing Kainite) is obtained only after two earlier solid evaporates, i.e., crude salt and Sels mixture are removed separately. This is done by solar evaporation in pans, removal of salts from pans, and pumping of liquid into intermediate pansxe2x80x94all of which are highly labor and energy intensive. In order to produce these salts the bittern has to be concentrated to densities as high as 37.5xc2x0 Bexe2x80x2 (Sp. Gr. 1.348) which requires longer evaporating period and/or larger area. Secondly, kainite type mixed salt is to be processed further by mixing the same with high-density bittern and using hot extraction technique followed by cooling to extract carnallite from mixed salt. This is a tedious operation and involves high-energy consumption accompanied by loss of potash in various effluent streams. Thirdly, there is considerable loss of valuable magnesium in all the solid evaporates and there is no provision in this process to recover other products like high purity magnesia.
Reference may be also made to the articles by M. K. Raval and K. V. Satyanarayana in xe2x80x9cBromine Content in Bittern From Salt Works in Kudaxe2x80x94Kutch Regionxe2x80x9d, Salt Research and Industry, Vol. 4, No. 2, (April 1967), 56-58 and by M. H. Jadhav and V. V. Chowgule in xe2x80x9cBromine concentration with rise in Density of Sea Bitternxe2x80x9d, Fifth International Symposium on Salt, from which it can be concluded that although there is increase in bromide concentration in bittern with evaporation, a significant part of the original bromide content in bittern tends to be lost in the solid evaporates in the course of recovery of potash via kainite salt as described above. This constrains bromine to be recovered at 29-32xc2x0 Bexe2x80x2, as a result of which its recovery is less efficient, since bromide concentration in bittern is in the range of 2-4 gLxe2x88x921.
Reference may be made to xe2x80x9cImproved Treatment of Waste Brinesxe2x80x9d by Chr. Balarew, D. Rabadjieva and S. Tepavitcharova, (International Symposium on Salt (2000), 551-554) for recovery of marine chemicals. The principle drawbacks of this process, which advocates crystallization of salt followed by removal of magnesium in bittern with lime, subsequent recovery of potash and recirculation of calcium chloride into bittern for the purpose of desulphatation, are that salt quality is not improved in any way, and the recovery of potash would involve removal of large quantities of water which is not feasible with solar evaporation.
Reference may be made to xe2x80x9cPotassium chloride from sea bitternxe2x80x94Part 2, Recovery of potassium chloride, magnesium sulphate and potassium sulphatexe2x80x9d by Gadre G. T., Rao A. V. and Bhavnagary H. M., Jr. Sc. Ind. Res., 17(A), 9, (1958), 376, wherein bittern is cooled to 10xc2x0-5xc2x0 C. to crystallize sulphate ion as Epsom salt. The bittern, after removal of sulfate, is concentrated to crystallize carnallite. The main drawback of this process is that apart from high cost of refrigeration and bulk handling, the process removes sulphate to a maximum extent of 50% of sulphate originally present in bittern, which at a later stage will contaminate carnallite, rendering the product impure.
According to the present invention, desulphatation of low density brine, i.e., brine prior to crystallization of salt, with in situ generated calcium chloride or with calcium chloride in distiller waste is found to be a highly effective solution to all of the drawbacks described in the prior art. It has been found in the course of the invention that, although calcium impurity in salt is among the principal concerns which dictates its price and usability in chloralkali industry, the addition of calcium chloride to brine for the purpose of desulphatation does not increase calcium impurity of salt but actually decreases it. Without wishing to be bound by any theory, it is believed that addition of calcium chloride forcibly eliminates calcium sulphate as a precipitate because of the large calcium ion concentration in brine and the sparing solubility of calcium sulfate. As a result, less calcium sulphate coprecipitates with common salt during the crystallization of the latter at 25xc2x0 Bexe2x80x2 and beyond. The residual calcium in the brine which adheres to the salt crystals is easily washable as it is primarily in the form of calcium chloride, which has much higher solubility than calcium sulphate. Removal of sulfate also reduces build up of magnesium sulphate impurity in salt, and the adhering magnesium chloride impurity is easily washable. Most remarkably, sub-soil brine, which yields salt of the lowest purity, is especially attractive since the requirement of desulphating chemical is the least and the salt quality can be upgraded to purity even superior to that obtained presently for sea salt. As further established in the course of the invention, addition of calcium chloride to effect desulphatation does not in any way deteriorate the characteristics of the bittern, and camallite can be recovered with ease. Further, as found in the course of the present invention, desulphatation also allows steady build-up of bromide concentration in bittern with negligible loss in solid evaporates. Furthermore, desulfatation allows high purity magnesium chloride to be formed, a part of which can be converted into magnesium oxide of high purity with concomitant production of hydrochloric acid, which can be utilized for production of calcium chloride. Another novelty of the present invention is the use of soda ash distiller waste for desulphatation. Such waste is rich in calcium chloride and sodium chloride, both of which are useful in the methodology of the invention.
The present invention provides an improved and integrated process for recovery of salt and marine chemicals, which is centered around desulphatation of brine and obviates the drawbacks as detailed above.
The present invention also provides a method for preparing high purity salt, particularly from sub-soil brine, through simple washing of the crystallized salt with water, and at virtually no extra cost, through the process of integration, and further to prepare salt of very high purity through only additional incremental cost.
The present invention also provides a method for integrating salt manufacture with soda ash production, and for using calcium rich distiller waste generated in soda ash plants for the desulphatation process.
The invention also provides a method for recovering salt and marine chemicals from high density, low sulphate sub-soil brine so as to maximize salt productivity, minimize requirement of desulphating chemicals, and achieve the highest differential improvement in salt quality.
The present invention also provides a seeding process for easy granulation of calcium sulphate leading to the easy separation from brine.
The present invention also provides a method for using the carbon dioxide gas produced when lime stone is dissolved in hydrochloric acid, to produce magnesium carbonate and potassium carbonate in the down stream processes, through well established processes.
The present invention also provides a method for determining that there is negligible loss of bromine when desulfated bittern is processed by further evaporation to produce carnallite, with the result that bromide can be enriched in the end bittern and can then be processed by the well established methods of bromine recovery which prefer higher concentration of bromide ions for better economy.
The present invention relates to recovery of industrial grade salt and marine chemicals from brine in an integrated manner. The process involves treatment of salt brine with calcium chloride to precipitate calcium sulphate, solar evaporation of desulfated brine in crystallizers to produce salt, solar evaporation of bittern to produce carnallite, decomposition of carnallite to recover sodium chloride and potassium chloride mixture and processing of this solid mixture to produce potassium chloride by known hot extraction technique. End bittern obtained after crystallisation of carnallite is calcined to produce high purity magnesia and hydrochloric acid. Limestone is treated with hydrochloric acid to produce calcium chloride, which is recycled for desulphatation of brine while the carbon dioxide can be recycled for preparation of carbonates of magnesium and potassium by well established routes.
Accordingly, the present invention provides a process for recovery of common salt and marine chemicals from brine 3-240Bexe2x80x2 in integrated manner comprising the steps of:
(i) reacting of 1-12 M hydrochloric acid which is obtained from calcination of magnesium chloride of end bittern at 600-800xc2x0 C. with calcerous material including limestone in stoichiometric ratio of one part of limestone with two parts of hydrochloric acid to prepare calcium chloride of 100-600 g/L concentration required for desulphatation;
(ii) treating said brine with calcium chloride as obtained in step (i) to produce granular calcium sulphate through a seeding process;
(iii) separating calcium sulphate from brine;
(iv) evaporating desulphated brine in solar pans up to 29-32xc2x0 Bexe2x80x2 thereby crystallising out salt;
(v) washing salt with water or dilute brine to remove adhering chlorides of calcium and magnesium;
(vi) evaporating bittern in solar pans from density range of 29xc2x0 Bexe2x80x2 to 35.5xc2x0 Bexe2x80x2to crystallise crude carnallite and thereafter recovering potassium chloride by known techniques;
(vii) recovering concentrated end bittern comprising magnesium chloride and enriched bromide; and
(viii) solidifying a part of the end bittern and calcining in the temperature range of 600-800xc2x0 C. to produce solid magnesium oxide and hydrochloric acid sufficient for recycling in step (i).
In an embodiment of the present invention, calcium chloride, as in distiller waste of soda ash in the concentration of 5-15% CaCl2 in 0.8-1.2 mole of calcium to sulphate, can also be used.
In another embodiment of the present invention, treating the desulphated brine as obtained in step (ii) above with barium chloride in 0.80-0.95 mole ratio of barium to residual sulphate ion can be used to ensure near-complete desulphatation.
In still another embodiment of the present invention, marine chemicals such as common salt, potassium chloride, magnesium chloride enriched with bromide, high purity magnesia and calcium sulfate with  less than 0.5% chloride, can be produced in an efficient and integrated manner from sub-soil/sea brine of 3-24xc2x0 Bexe2x80x2 density and sulphate concentration typically in the range of 5-18 g/L measured at 16xc2x0 Bexe2x80x2.
In yet another embodiment of the present invention, recovery of said marine products can be most efficiently undertaken through reduction of sulphate concentration of brine to a concentration in the range of 0.5-2.0 g/L.
In still another embodiment of the present invention, the reduction of sulphate is achieved by adding calcium chloride produced in situ.
In yet another embodiment of the present invention, removal of calcium sulphate from desulfated brine is facilitated through a seeding technique which allows easy granulation of the resultant calcium sulphate formed.
In still another embodiment of the present invention, sub-soil brine having high sodium chloride concentration (up to 18xc2x0 Bexe2x80x2) and low sulphate concentration ( less than 6 g/L at 16xc2x0 Bexe2x80x2) is especially suitable as brine source.
In yet another embodiment of the present invention, brines located in the vicinity of soda ash plants can be treated with the distiller waste containing 5-15% calcium chloride.
In still another embodiment of the present invention, the primary process of desulfatation, salt recovery and carnallite production can be carried out readily in the field in large solar pans.
In yet another embodiment of the present invention, desulfatation allows build up of bromide concentration in bittern up to 7.5 g/L at 35.5xc2x0 Bexe2x80x2 without any significant loss of bromide along with crystallized solids during evaporation.
Calcium chloride can be prepared by reaction between limestone and recycled hydrochloric acid in leach tank under ambient condition followed by treatment with a small amount of lime to raise pH to 5.5 and filtration through a bed of calcium sulphate produced in the process itself to eliminate unwanted color from iron impurities. The concentration of calcium chloride solution is preferably maintained between 410 and 440 gLxe2x88x921. Calcium chloride is alternatively obtained as a clear liquid after settling the distiller waste from soda ash, which typically has a composition of 10-12% CaCl2 and 5-7% NaCl.
Brine, preferably with density in the range of 15xc2x0 Bexe2x80x2-22xc2x0 Bexe2x80x2 (Sp. Gr. 1.11-1.14), is treated with calcium chloride solution to eliminate calcium sulphate as described above. This reaction can be carried out in a reaction vessel or preferably in the field in large solar pans. When undertaken in a reaction vessel, a part of the output slurry of calcium sulphate is fed back to the vessel as seed. This makes the precipitate granular, which settles efficiently at the bottom.
Desulfated brine is allowed to concentrate in the condenser and then charged into the crystallizer at around 25xc2x0 Bexe2x80x2 (Sp. Gr. 1.21) whereupon common salt crystallizes out. Evaporation of brine treated thus in solar pans yields salt of high purity when washed with small quantities of dilute brine or fresh water in the field to remove adhering calcium and magnesium chlorides.
Desulfatation with calcium chloride does not remove sulphate completely from the brine, and small amount of calcium in the form of calcium sulphate coprecipitates with the crystallized salt. If salt of still higher purity is required, more complete desulphatation of brine may be carried out with barium chloride whose usage, however, is minimized because of the first stage of desulphatation with calcium chloride. To avoid any contamination of the salt with barium ion, the barium chloride is used in slightly less than the stoichiometric amount of sulphate present in the brine following treatment with CaCl2, the sulphate concentration typically being in the range of 1-3 g/L at 24xc2x0 Bexe2x80x2. It is preferable to carry out desulphatation with barium chloride in reaction vessels than in the field.
The mother liquor (bittern) obtained after crystallization of salt having density in the range of 29-30xc2x0 Bexe2x80x2 (Sp. Gr. 1.25-1.26) is fed to shallow impermeable solar pans where it undergoes further solar evaporation. As density rises to 32 to 33xc2x0 Bexe2x80x2 (Sp.Gr. 1.28-1.284), excess sodium chloride present in original bittern crystallizes out, which is removed. On further evaporation, camallite double salt (KCl.MgCl2.6H2O) crystallizes out at a density of 35 to 35.5xc2x0 Bexe2x80x2 (Sp.Gr. 1.318-1.324) along with residual NaCl as well established in the art.
Carnallite is decomposed with water to remove magnesium chloride and a mixture of potassium chloride and sodium chloride. Purification of the latter to obtain potassium chloride is achieved as well established in the prior art. Residual sodium chloride/potassium chloride is fed back into the carnallite pan for enhanced recovery in the subsequent cycle.
Bittern obtained after removal of carnallite, having a density of up to 35.5xc2x0 Bexe2x80x2 (Sp. Gr. 1.324), is a concentrated solution of magnesium chloride and is known as end bittern with magnesium chloride concentration ranging from 400 to 430 gLxe2x88x921. The end bittern was analyzed for bromide and its concentration found to be 7.5 gLxe2x88x921 (expressed as elemental bromine), i.e., nearly 3 times the bromide concentration at 29xc2x0 Bexe2x80x2 (Sp.Gr. 1.25) and 1.5 times the concentration at 32xc2x0 Bexe2x80x2 (Sp.Gr. 1.28) which is typically the density range at which bromine is recovered in many plants. Since the volume of bittern is reduced by a factor of three in going from 29xc2x0 Bexe2x80x2 to 35.5xc2x0 Bexe2x80x2, there is essentially no loss of bromide during the process of concentration.
End bittern is reacted in calcination system at a temperature ranging from 600 to 800xc2x0 C. to form magnesium oxide and hydrochloric acid according to the following equation:
MgCl2.6H2Oxe2x86x92MgO+2HCl+5H2O 
The following examples are given by way of illustration and should not be construed to limit the scope of the invention: